How to install from source?

What if i'm getting "undefined symbol"

If your php is configured --enable-versioning, disable it. It's broken. If you insist on versioning, build XCache as a static module of php.

Apache2 keeps getting segmentation faults with cache larger than 4M

Set "xcache.readonly_protection = On" in php.ini

I get "Cannot dynamically load xcache.so"

When u get PHP Warning: Unknown(): Cannot dynamically load xcache.so - dynamic modules are not supported in Unknown on line 0, it's because your php is running on a arch/os that don't support dynamic load library(aka modules), to solve it:

Why is misses > 0 no matter how I increase the shm size?

"Misses" a opp to "hits". You get cache misses when there's nothing in the cache in the first time, after when it is cached and you starts getting cache hits. So there shall be at least misses=count(cached-php).

Why is my cache being cleared?

Set PHP_FCGI_CHILDREN to 1 or greater if you're using php FastCGI.

And see the following.

I see jumpy "hits" in admin page

It is possible that u see jumpy "hits" count in admin page. i.e.: you see hits=300,000 on first load of admin page, but dropped to hits=1 in 2nd reload and back to hits=312,456 on 3rd load.

This is because you have php started in different group which does not share shm with each other. But keep in mind that all childs in 1 group will share shm. So how to check and make it 1 group only?

xcache.so: undefined symbol: core_globals_id

It can be *_globals_id or *_id. This error mean XCache was built with wrong PHP headers (the include/php/*.h files), either it was wrong version or wrong PHP built option, like 4.x vs 5.x or PHP zts built vs non-zts built. Apparently, it can be fixed easily by removing the wrong headers and rebuild XCache.

Remove wrong PHP headers
PHP headers can be php-devel package on some system, simply reinstall or upgrade it. But if your php is installed manually, try:

Rebuild XCache

I got error: * is incompatible with XCache n Unknown on line 0

Use zend_extension=/path/to/xcache.so or zend_extension_ts=..., and make sure it's before any other zend_extension=...

I have read that xcache.optimizer setting gives an important performance gain. is that true ?

Currently only the "cacher" and "coverager" modules are implemented, tested and known to be working, the "optimizer" does nothing. it will be available only in XCache version 2, which is in an early development stage.